<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .old-box {
            width: 500px;
            height: 400px;
            background-color: orchid;
            text-align: center;
            line-height: 400px
        }

        .old-item {
            width: 100px;
            height: 100px;
            color: #fff;
            text-align: center;
            line-height: 100px;
            display: inline-block;
            margin: 0;
        }

        .old-item1 {
            background-color: #f00;
        }

        .old-item2 {
            background-color: #0f0;
        }

        .old-item3 {
            background-color: #00f;
        }

        /* **************分割线************** */
        .new-box {
            width: 500px;
            height: 400px;
            background-color: orange;
            display: flex;
            justify-content: center;
            align-items: center
        }

        .new-item {
            width: 100px;
            height: 100px;
            color: #fff;
            text-align: center;
            line-height: 100px;
        }

        .new-item1 {
            background-color: #f00;
        }

        .new-item2 {
            background-color: #0f0;
        }

        .new-item3 {
            background-color: #00f;
        }
    </style>
</head>

<body>
    <div class="old-box">
        <div class="old-item old-item1">左</div>
        <div class="old-item old-item2">中</div>
        <div class="old-item old-item3">右</div>
    </div>
    <br>
    <div class="new-box">
        <div class="new-item new-item1">左</div>
        <div class="new-item new-item2">中</div>
        <div class="new-item new-item3">右</div>
    </div>
</body>

</html>